After fasting for 8 to 12 hours, the amount of glucose in the hepatic portal vein is markedly lower than the amount after a meal, but the amount in the systemic circulation remains near the normal level. Hepatocytes are able to maintain systemic
Glucose homeostasis in this situation primarily by gluconeogenesis and which of the following biochemical mechanisms?

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Glycogenolysis is the process by which glycogen stored in the liver is broken down into glucose, which is then released into the bloodstream to maintain normal blood glucose levels during fasting.  During periods of fasting, the pancreas releases glucagon to stimulate the liver to deliver glucose to the blood. When glucagon binds to its receptors on hepatocytes, there is activation of adenylate cyclase resulting in increased cAMP with consequent activation of PKA. PKA phosphorylates and activates glycogen synthase-phosphorylase kinase, which will phosphorylate and activate phosphorylase while also phosphorylating and inhibiting glycogen synthase. The increased activity of phosphorylase results in increased release of glucose from glycogen and its subsequent release to the blood.

After Fasting for 8 to 12 Hours, the Amount of Glucose